System for measuring vibrations occuring on a surface

ABSTRACT

A device and system is provided for amplifying vibrations resulting from underwater acoustic signals. In operation, a laser interrogation beam is directed along an axis at the retro-reflector device and is responsive to reflections directed along the axis of the interrogation beam. The retro-reflector device reflects a signal back to a source and a tracking signal superimposed on an interrogation beam enables continuous sensing of the reflected signal to reduce signal dropout. A glint tracker is provided for steering the tracking beam on the surface. A tracker system superimposes the tracking and interrogation beams and is responsive to reflected glints in order to establish a directional location. An interferometer responsive to the reflected interrogation beam produces an interference signal for enabling continuous measurement of surface vibrations.

This application is a divisional of pending prior U.S. patentapplication Ser. No. 12/902,571 filed on Oct. 12, 2010 entitled “LaserBased Acousto-Optic Sensing Enhancement Using TailoredRetro-Reflectors”. This application claims the benefit of the priorapplication's filing date.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for Governmental purposeswithout the payment of any royalties thereon or therefore.

CROSS REFERENCE TO OTHER PATENT APPLICATIONS

None.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention is directed to a sensing and tracking system andmethod of use for enhancing laser based acousto-optic sensing with thesensing and tracking system employing tailored retro-reflector devices.The retro-reflector devices amplify acoustics sensed in particularfrequency bands as a function of an acoustic incidence angle and a laserbeam interrogation angle. The sensing and tracking system searches foroptical reflections or glints from one or more deployed retro-reflectordevices in order to measure surface vibrations of the devices. Thesurface vibrations are caused by the amplified acoustics of anunderwater sound source incident upon the devices.

2) Description of the Prior Art

A laser Doppler vibrometer (LDV) is a commonly-known device which iscapable of directing a single output laser beam onto a measuring surfacein which surface vibrations are measured when the laser beam isreflected back from the measurement surface into the LDV. Interferencebetween the output laser beam and the reflected beam provides ameasurement of the velocity and displacement of the surface. This actionis useful for measuring surface vibrations produced by pressure wavesfrom an underwater source.

However, reflections from a water surface tend to be weak, sporadic, andthus difficult to detect. A surface with poor reflective quality,turbulence, a high sea state or a foamy condition degrades sensorperformance by deflecting the laser beam away from the detector;thereby, increasing signal dropout. One remedy is to monitor the watersurface glint locations and to continually steer the laser beam onto anoptical glint—such as a naturally direct reflecting point on the unaidedwater surface.

Glint detection is accomplished by directing a laser at the watersurface; identifying areas of direct reflection back to the source;obtaining an image of these temporally and spatially varying laser glintpositions using a photosensor array; and steering the beam into theglint location on the water surface. In a real world example, laserbased tracking systems are employed during eye surgery to accommodateeye movement during the operation.

A combined LDV and tracker system finds the points where reflection willoccur by employing a complex algorithm to continually steer the lightbeam of the tracker in order to maintain sight of the dynamic reflectionpoint on the water surface. An issue with this system is that naturalwater surface glints depend on sea state wave slopes and predominantlyoccur at a nadir+/−20°. This limits the angles at which the laser can beemployed to probe the water surface.

Since the water surface acts as a specular reflector, the output laserof the LDV must be perpendicularly incident to the water surface inorder to acquire a reflected beam. In normal operating situations,turbulent and hydrodynamic conditions prevail; thereby, resulting insignificant intermittence of a received optical signal.

Intermittence of signal detection occurs when the slope of the wavesurface changes with respect to the incident laser beam angle. The mosttroublesome problem governing LDV performance on moving reflectivesurfaces is signal dropout. In such a situation, it is difficult for thesensor system to capture the reflected beam.

Another problem is the poor reflective quality of the surface. Oneremedy is to illuminate the surface and to track the reflections.Image-based tracking using an array of photosensors finds points wherethe required reflection will occur in the tracker which seeks to findthe point of reflection and which relies on complicated algorithms tosteer the laser beam onto this dynamic point.

Natural water reflections or glint points that are identified by anilluminating source, predominantly occur at the nadir+/−20°. Thisoccurrence limits the angles that the laser output beam may probe thewater surface. Also, glint features on the water surface tend to betemporarily and spatially indeterminate which also makes continuoustracking difficult.

An example of a system for tracking glints is disclosed in U.S. Pat. No.7,251,196, (Antonelli et. al.) and is commonly assigned to the assigneeherein. The teachings of U.S. Pat. No. 7,251,196 are incorporated hereinby reference.

The Antonelli reference discloses a passive acoustic sensor to detectunderwater sounds by using optics in order to determine vibration on theunaided surface. The tracking system must produce a light beam that isperpendicularly incident to the water surface. This positioning reducesthe angular approaches that are available for tracking.

In addition, the turbulent and hydrodynamic wave conditions that oftenprevail under normal situations cause the slope of the turbulent wavesurface to rapidly change. Accordingly, significant intermittence orsignal dropout of the optical signal reflected from the water surface isexpected as the slope changes relative to the incident angle of thelaser beam.

A known laser-pumped acoustic sensor system is disclosed in U.S. Pat.No. 7,113,447 (Matthews et al.) and assigned to the assignee herein. Theteachings of U.S. Pat. No. 7,113,447 are incorporated herein byreference. The system of the cited reference discloses a laser-pumpedcompact acoustic sensor system, wherein one or more hollow sphericalshells vibrate in response to impinging acoustic signals. The shellshave one or more portions that are reflective of impinging laserradiation. A resilient matrix, in contact with the water, supports theshell.

Further, in the cited reference, a laser Doppler velocimeter transmitsradiation onto the reflective portion of the shell and receivesreflected radiation therefrom. The reflected radiation produces signalsin the laser Doppler velocimeter that claim to be representative ofacoustic signals in the water. A computer, responsive to the signals,produces a display representative of direction and range to a target.

Known optical sensor systems rely on reflections from a reflectingobject. However, the reflections are not directional; meaning that thediffuse reflections reflect energy in all directions and only a smallportion of the reflected energy is detected. Accordingly, these systemsare susceptible to background light noise; thereby, resulting in reducedreliability.

There is therefore a need for a system to amplify, locate, track anddetect laser reflections with increased temporal and spatial resolution.Such a system should be able to track and detect reflections overbroader interrogation angles; detect reflections with reduced or nosignificant signal dropout; and increase the signal strength ofreflected signals.

SUMMARY OF THE INVENTION

The present invention generally relates to the use of and supportingsystem for a retro-reflective device deployable on a water surface. Theretro-reflective device is capable of reflecting a beam of laserradiation back to a source along the incident beam angle. Theretro-reflector device amplifies resonant vibrations imparted thereto bypressure waves emanating from an underwater source of acoustic radiation(ie: a target).

The retro-reflector device is employable with the supporting system fortracking and detecting tracking optical beam reflections or glints of atracking beam directed towards a field having one or moreretro-reflector devices. By use of the retro-reflector device, theglints are sustainable through changing wave slope conditions; thereby,facilitating tracking and reducing signal dropout of laser reflections.The reflected laser radiation is combined in an interferometer with thereference beam to determine the amplitude and frequency of the surfacevelocity and displacement due to the underwater source of acousticradiation.

The retro-reflector device comprises a shell having physical propertiesfor controlling a characteristic spatial and spectral response toacoustic signals. Physical properties of the shell include a thickness,radius and material composition which are chosen to affect spatial,resonance, and spectral response of acoustic signals. The resonantvibration frequency of a hollow, spherical device occurs when the shelldiameter is half of the acoustic wavelength.

According to one embodiment of the invention; the retro-reflector devicecomprises an evacuated rigid shell (such as a sphere or cylinder). Theshell exhibits a resonance such that a free field surface velocity ofthe retro-reflector device is greater than twice the particle velocityof the water by the condition that the water-to-retro-reflector boundaryconstitutes a pressure release surface.

A spherical retro-reflector device having a selected thickness, radiusand material composition can amplify impinging acoustic signals byvibrating at or near its resonant frequency. The resonant vibrationamplitude is distributed spatially along the outer surface of theretro-reflecting sphere. Therefore, the measured vibration amplitudewill vary as a function of an acoustic incidence angle distributedspatially across the outer surface of the retro-reflecting spheredevice. Therefore, the measured vibration amplitude will vary as afunction of an acoustic incidence angle distributed spatially across theouter surface.

The retro-reflector device has at least one frequency resonant acousticresponse greater than an incident plane wave velocity that is specificto the spherical device dimensions; a spatially-distributed response onthe outer surface from an acoustic incidence angle selective responsedue to constructive and destructive interference of the device's surfacevibrations. As such, the measured vibration amplitude may vary as afunction of the interrogation beam angle.

The retro-reflector device includes one or more retro-reflectingsurfaces with approximately one hundred percent reflectivity. Theretro-reflectivity is in a range of the nadir+/−90°. The retro-reflectordevice also enables a sustainable glint for as long a duration as isvisible to the tracker system and laser interferometer interrogationbeam.

The retro-reflector device has a deterministic shape, such as hollowsphere, for enhancing temporal and spatial trackability; and produces aminimum detectable signal at least greater than that of a free surface.

The retro-reflector device has a free field surface velocity greaterthan two times the in-water particle velocity and is governed by one ormore of: a resonant frequency, f^(n) _(res), wherein “n” is at leastone; a shell damping loss factor “ε” of approximately “j” modelasticity/real mod elasticity governing peak V_(amp) and sharpness ofresonance frequency, and where ε is inversely proportional to V_(amp)and is directly proportional to resonance width.

The retro-reflector device has a f^(sub) _(res)<=f^(float)_(res)</=f^(vac) _(res), where: f^(vac) _(res) is invacuo resonance;f^(sub) _(res) is fully-submerged resonance; and f^(float) _(res) is onehalf of submerged resonance for the device. Estimated solutions forf^(sub) _(res); f^(float) _(res); and f^(vac) _(res) are governed by anexpression f^(sub) _(res)≈(f^(vac) _(res)+f^(sub) _(res))/2.

In another embodiment, the invention comprises a system for measuringvibrations occurring on a surface having at least one retro-reflectivedevice; a glint tracker for producing a scanning tracking beam on anaxis in order to locate the retro-reflector device; and a detectorresponsive to reflected glints along the axis from the retro-reflectordevice in order to establish a directional location thereof.

The system also comprises a laser interferometer (such as a laserDoppler vibrometer) for producing a coherent laser interrogation signalalong the axis of the tracking beam directed at the retro-reflectordevice and for being responsive to detect reflections of theinterrogation signal from the retro-reflector device in order to enablecontinuous measurement of surface vibrations thereof.

In yet another embodiment, the invention is directed to a method formeasuring acoustic vibrations occurring on a surface with the methodcomprising the steps of: deploying one or more retro-reflective devicesonto the water surface; producing a tracking beam on an axis; detectingreflected tracking beam radiation from the retro-reflector devices alongthe axis; establishing a directional location; directing a coherentlaser interrogation signal along the axis of the tracking beam;detecting reflections of the interrogation signal from theretro-reflector devices; and measuring surface vibrations of theretro-reflector devices based on the reflections.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustrating an arrangement of the presentinvention; and

FIG. 2 is an illustration of the use of a retro-reflector deviceaccording to the present invention.

DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a system 10 of the present invention in which thesystem is capable of detecting acoustic signals 12 produced by anundersea sound source 14. In operation, the acoustic pressure wavesgenerated by the sound source 14 impinge on retro-reflector devices 20deployed over a search field 24 and floating on an ocean surface 26. Theretro-reflector devices 20 are capable of vibrating in response to theimpinging pressure waves of the acoustic signals 12.

A laser interferometer 36 produces an output interrogation beam 38 thatis directed to a beam steering device 40.

The interferometer 36 may be any one of a variety of suitable devicesknown in the art for measuring the change in the optical characteristicof the interrogation beam reflected from the retro-reflector device 20.In the exemplary embodiment, the interferometer 36 illustrated is alaser Doppler vibrometer (LDV).

An infrared (IF) source 42 produces a tracking beam 44 which is directedto the steering device 40. The interrogation beam 38 and the trackingbeam 44 are superimposed on each other in the steering device 40. Thesteering device 40 then transmits the combined laser interrogation beamand tracking beam along a beam axis “A”, at an interrogation angle θtowards the targeted retro-reflector device 20.

The retro-reflector devices 20, 20′ amplify the pressure waves impingingthereon from the sound source 14 by producing resonance vibrations in aparticular frequency range on an outer surface 46 of the device. Themagnitude of the detected vibrations is a function of the acoustic soundpressure level, the acoustic incidence angle θ and the wavelength of thelaser interrogation beam 38.

The retro-reflector device 20 is characterized by the ability to reflectan incident optical beam back toward its source. Retro-reflectormaterials include any material that has retro-reflective capability. Inthe exemplary embodiment, the retro-reflector device 20 is a hollowbrass sphere having a retro-reflective outer surface coating. Surfacetreatments include paint, tape and the like in which the treatments haveretro-reflective properties.

In the present arrangement, a combined interrogation and tracking beam(hereinafter “47” as the combined beam) are reflected back towards thesteering device 40.

The reflected tracking beam (hereinafter called glint 48) is sensed by aphoto-detector contained within a beam steering controller system 50which senses glints from the retro-reflector device 20 and controls thesteering mirrors within the beam steering device 40 in order tocontinuously track the position of the retro-reflector device. Thesteering device 40 is capable of steering the optical axis “A” of thecombined interrogation and tracking beam 47 to follow theretro-reflector device 20 that is being tracked.

The combined interrogation and beam 47 is reflected back from theretro-reflector device 20 back towards the steering device 40 and isthereafter directed along an optical path 52 to an input of theinterferometer 36. The reflected interrogation beam is directed onto thephoto-sensor within the interferometer 36 along with a reference laserbeam contained within the interferometer system to produce a signalindicative of the velocity of the amplified vibrations produced by theretro-reflector device 20. This signal is an indicator of the amplitudeand frequency of the underwater sound source 14.

When the combined beam 47 is directed at two or more retro-reflectordevices 20′ to detect a target signal; the signals may be employed toprovide a more accurate position of the underwater sound source 14 usingtraditional beamforming analysis combined with knowledge of the steeringangle of the axis “A” of the beam steering device 40.

The combined beam 47 is scanned over the search field 24 by the steeringdevice 40 and searches for glints from the retro-reflector devices 20,20′. Detected glints are then tracked by the steering control 50 and thesteering device 40 to direct the beam 47 to illuminate theretro-reflector devices 20, 20′ and returns the reflected interrogationbeam contained in 48 to the interferometer 36 via the steering device 40and the optical path 52.

The reflected interrogation beam on the optical path 52 is combined witha reference laser beam within the interferometer 36 to produce amplifiedoutputs indicative of the acoustic pressure waves. A measurement of theamplitude and frequency of the surface is obtained from which theposition of the underwater sound source 14 is derived within a systemanalyzer and controller 51. The system analyzer and controller 51obtains the voltage output from the interferometer 36 and the steeringangle from the tracking controller 50. The system analyzer andcontroller 51 calculates the sound source signal amplitude and frequencycontent including the vibration velocity and displacement and location.

The tracking feature allows the superimposed interrogation beam 47 toclosely follow the retro-reflector device 20 whereby continuousmeasurement of the reflected signals may be obtained with highresolution and little or no signal dropout.

Naturally-occurring glints are temporally unstable and tend to quicklydisappear within approximately one millisecond—depending on the seastate conditions. As a result, the tracker must acquire a new glint. Theretro-reflector device 20 provides a more sustainable glint that can beefficiently and sustainably probed with the laser beam.

As previously noted, the foremost issue governing acousto-optic sensorperformance on moving reflective surfaces is signal dropout due to laserreflections not being captured by the sensor system or beam detector.Optical dropout that is prevalent with conventional systems isdrastically reduced because the retro-reflector device 20 can float onthe water surface; can self-right itself and can provide a stable, wideangle reflection of the interrogation and tracking beams at oblique andnormal incidence. The ability of the retro-reflector device 20 toreflect back along the same optical path as the source, allows for awidened range of laser probe angles which can greatly reduce signaldropout.

The retro-reflector device 20 described herein moves on the watersurface in a more determinable and predictable way as compared with aglint feature from a natural or an unaided water surface. Theretro-reflector device 20 also has a deterministic shape that is easilyrecognized and is readily trackable temporally and spatially than in animage-based, laser glint tracking system—operating on an unaided oceansurface.

The retro-reflector device 20 can resonate in order to amplify theacoustic vibrations impinging on the device; thereby, presenting astronger signal for detection. Specific features are designed into theretro-reflector device 20 to increase sensitivity within certainacoustic frequency bands as a function of the acoustic incidence angleand laser beam interrogation angle.

The minimum detectable signal level as well as detection sensitivity areincreased by several orders of magnitude. The combination of theinterferometer 36 with distributed retro-reflector devices 20 results inan improved system with enhanced performance.

A tracking and detection system according to the present inventionemploys retro-reflector devices; a laser interferometer; and a glinttracking system. In operation, the interferometer produces an outputbeam directed along a steerable beam axis. The output beam is directedto one or more locations of the retro-reflector devices distributed in asearch field on the ocean surface.

A continuous wave (CW) laser source or tracking beam is used to scan thesurface in order to locate a glint location and to direct theinterrogation beam 47 at the retro-reflector device 20 in order todetect the reflected light containing information of the amplifiedacoustic response of the retro-reflector device.

The interrogation beam 38 is superimposed on the tracking beam 44 and issteered by the steering device 40 to continually track reflected glintsfrom the retro-reflector devices 20 so that signal dropout is reduced oreliminated.

An exemplary retro-reflector device 20 is illustrated in FIG. 2. In thefigure, the retro-reflector device 20 comprises an evacuated brasssphere having a diameter “D” of approximately 1.5-5 inches and a wallthickness “T” of approximately 1.5 inches. The retro-reflector device 20may also be cylindrical or any other suitable shape.

The retro-reflector device 20 may be formed of other metals andnon-metals (such as plastics and ceramics) having suitable resonanceproperties for amplifying the acoustic vibrations impinging fromsub-surface acoustic sources.

The retro-reflector device 20 in FIG. 2 is depicted as a half-submergedsphere on the ocean surface 26. The retro-reflector device 20 has anin-air section 70 and an in-water section 72. The parameter choicescontrol the spatial and spectral magnitude (i.e., amplification,structure of the acoustic resonance properties) of the exposed in-airsection 70 which are a function of the acoustic angle of incidence Φ ofan incident acoustic wave (incident plane wave). In operation, thecombined beam 47 probes the retro-reflector device 20 along the axis “A”and receives a reflected beam from the in-air section 70 along the sameaxis.

For a half-submerged floating elastic shell, an incident acoustic wave12 having a particle velocity V_(INC) that is impinging on the in-watersection 72 is amplified. With no retro-reflector device 20 present, thefree field surface particle velocity V_(SURFACE) is double the incidentparticle velocity (i.e., V_(SURFACE)=2V_(INC)).

The floating retro-reflector device 20 excites a resonance of the shellin order to magnify the velocity of a dry shell above the 2V_(INC) levelso that an acousto-optic sensor can more easily sense the signal carriedby the beam 38 reflected from the exposed upper in-air section 70.

It is generally desirable for the shell to amplify the incident planewave velocity V_(INC). In an exemplary embodiment, it is desired thatthe shell meet the criterion: V_(amp)=V_(shell)/V_(INC)>>2 foramplification of an incident acoustic pressure wave.

Exact analytical solutions are not currently available for cylindricaland spherical shaped models to define conditions under which significantfloating shell velocity amplification, V_(INC)>>2. However, simplifyingassumptions may be employed to establish a boundary on determining thepotential magnitudes of V_(amp) and importantly, at which incident wavefrequencies f_(res) where these amplifications can be expected.

Simulations of a submerged spherical retro-reflective shell show that asubstantial amplification of V_(INC) greater than two can be achieved bysynthesizing the retro-reflective shell to have resonant frequency f^(n)_(res) values corresponding to a low number of mode shape lobes(n=number of lobes).

Shell damping or loss factor, (H=ratio of complex-to-real modulus ofelasticity) governs the peak velocity amplification V_(amp) and thesharpness of the resonance in frequency. A smaller eta provides a biggerV_(amp) amplification (a desirable feature), but narrower shapedresonance plots (an undesirable circumstance). Conversely, a bigger etareduces amplification which is undesirable, but widens the resonanceplots which is desirable.

In the case of an exemplary half-submerged floating spherical shellretro-reflector device, the values of resonant sphere frequenciesf^(float) _(res) (floating sphere resonance) for a particular mode shapeis bounded by the expression: f^(sub) _(res) (fully submergedresonance)</=f^(float) _(res)</=f^(vac) _(res) (invacuo resonance).

Analytical solutions exist for the invacuo resonance and the fullysubmerged resonance. Estimates exist for half-submerged resonance asfollows: f^(sub) _(res)≈(f^(vac) _(res)+f^(sub) _(res))/2.

In an example, a finite element model for a floating cylinder wasemployed to determine the response of floating objects with a circularcross-section. The model is affected by water impedance loading of thehalf-submerged portion of the model. The model determines if averagingapproximation works when the actual solution is known as a reference;and whether deformation and velocity shapes are affected by non-normalincidence.

Two half-submerged cylinder resonant models were simulated and computedwith a Finite Element Method (FEM) and compared against an approximateaveraging formula where n=4 lobe mode: f^(float) _(res)=23.3 using FEM &f^(float) _(res)=22.7 with averaging. N=G lobe mode: f^(float)_(res)=59.1 using FEM & f^(float) _(res)=59.7 with averaging.

The half-submerged cylinder was loaded with a plane wave input at fourdifferent acoustic angles of incidence. Substantial shell velocitieswere experienced at off normal acoustic incidence, and different partsof the in-air portion have peak velocities that depend on the acousticplane wave velocity.

The results achieved with exemplary retro-reflector devices indicatethat the design of the mechanical properties provides a way to magnifythe acousto-optic response of the retro-reflector device; thereby,effectively increasing the overall acousto-optic sensitivity. The designof the retro-reflector device 20 can enhance certain particularfrequency components while suppressing other components. The response ofa retro-reflector device 20 as a function of acoustic incidence anglemay be employed to add yet another capability.

The combination of an LDV with tracking and retro-reflector propertiesenhances detection capabilities; thereby, maintaining a continuous laserreflection back to the interferometer 36 from the retro-reflectormeasurement surface at any angle of interrogation.

The acousto-optic sensor described herein is capable of measuringvibrations of surfaces with high speed variations of the temporal andspatial deterministic retro-reflective surfaces in motion at a slowerrate than the corresponding water surface.

The use of the retro-reflector device described herein provides standoffdetection in instances where the acousto-optic sensing system is at lowaltitudes because laser-induced glints on the unaided water surface aremainly confined to +/−20° of normal incidence.

The invention employs an interferometer but is not limited to anyparticular design type. It is believed that the described methodologyfor finding a useful design is sufficient to enable one of ordinaryskill in the art to find a useful design for a particular application.

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed and illustrated in order to explain the nature of theinvention, may be made by those skilled in the art within the principleand scope of the invention as expressed in the appended claims.

What is claimed is:
 1. A method for measuring acoustic vibrationsoccurring on a water surface, said method comprising the steps of:providing at least one retro-reflector device with the at least oneretro-reflector device comprising a shell capable of exhibiting aspatial and spectral response to acoustic signals wherein the shell isan evacuated rigid shell having a selected thickness, radius andmaterial composition capable of amplifying impinging acoustic signals asa function of acoustic incidence angle distributed spatially across anouter surface; deploying the at least one retro-reflector device ontothe water surface; producing a tracking beam on an axis with a glinttracker; directing the tracking beam on the water surface toward a fieldhaving the at least one retro-reflector device; detecting reflectedglints from the retro-reflector device along the axis for establishing adirectional location; superimposing a laser interrogation signal alongthe axis of the tracking beam toward the retro-reflector device;detecting reflections of the interrogation signal from theretro-reflector device, with said detecting step provided by a detector;interfering with a laser interferometer, a reference signalcorresponding to the interrogation signal and the reflections from theretro-reflector device; measuring surface vibrations using the referencesignal and the laser interferometer to extract at least one of a surfacevibration amplitude, surface vibration frequency, surface vibrationvelocity and surface vibration displacement.